diff --git a/src/net/http/transport.go b/src/net/http/transport.go
index 258b912b0a..d08504d7ab 100644
--- a/src/net/http/transport.go
+++ b/src/net/http/transport.go
@@ -1199,6 +1199,9 @@ func (t *Transport) dialConn(ctx context.Context, cm connectMethod) (*persistCon
 		if resp.StatusCode != 200 {
 			f := strings.SplitN(resp.Status, " ", 2)
 			conn.Close()
+			if len(f) < 2 {
+				return nil, errors.New("unknown status code")
+			}
 			return nil, errors.New(f[1])
 		}
 	}
diff --git a/src/net/http/transport_test.go b/src/net/http/transport_test.go
index ad63cca5fe..b838fe9002 100644
--- a/src/net/http/transport_test.go
+++ b/src/net/http/transport_test.go
@@ -4269,3 +4269,71 @@ var rgz = []byte{
 	0x00, 0x00, 0x3d, 0xb1, 0x20, 0x85, 0xfa, 0x00,
 	0x00, 0x00,
 }
+
+// Ensure that a missing status doesn't make the server panic
+// See Issue https://golang.org/issues/21701
+func TestMissingStatusNoPanic(t *testing.T) {
+	t.Parallel()
+
+	const want = "unknown status code"
+
+	ln := newLocalListener(t)
+	addr := ln.Addr().String()
+	shutdown := make(chan bool, 1)
+	done := make(chan bool)
+	fullAddrURL := fmt.Sprintf("http://%s", addr)
+	raw := `HTTP/1.1 400
+		Date: Wed, 30 Aug 2017 19:09:27 GMT
+		Content-Type: text/html; charset=utf-8
+		Content-Length: 10
+		Last-Modified: Wed, 30 Aug 2017 19:02:02 GMT
+		Vary: Accept-Encoding` + "\r\n\r\nAloha Olaa"
+
+	go func() {
+		defer func() {
+			ln.Close()
+			close(done)
+		}()
+
+		conn, _ := ln.Accept()
+		if conn != nil {
+			io.WriteString(conn, raw)
+			ioutil.ReadAll(conn)
+			conn.Close()
+		}
+	}()
+
+	proxyURL, err := url.Parse(fullAddrURL)
+	if err != nil {
+		t.Fatalf("proxyURL: %v", err)
+	}
+
+	tr := &Transport{Proxy: ProxyURL(proxyURL)}
+
+	req, _ := NewRequest("GET", "https://golang.org/", nil)
+	res, err, panicked := doFetchCheckPanic(tr, req)
+	if panicked {
+		t.Error("panicked, expecting an error")
+	}
+	if res != nil && res.Body != nil {
+		io.Copy(ioutil.Discard, res.Body)
+		res.Body.Close()
+	}
+
+	if err == nil || !strings.Contains(err.Error(), want) {
+		t.Errorf("got=%v want=%q", err, want)
+	}
+
+	close(shutdown)
+	<-done
+}
+
+func doFetchCheckPanic(tr *Transport, req *Request) (res *Response, err error, panicked bool) {
+	defer func() {
+		if r := recover(); r != nil {
+			panicked = true
+		}
+	}()
+	res, err = tr.RoundTrip(req)
+	return
+}